Medical Equipment Radiotherapy1

1. Medical Equipment III
Radiotherapy

2. What is Cancer?
Cancers are growths of cells (cancerous tumors)
which are out of control. As a result of this, they
do not perform their intended function.

3. Normal and Cancer Cells

4. Treatment of Cancer
Cancerous tumors can be treated using the
following main methods:
 Chemotherapy (drugs).
 Radiation therapy:
 Radiotherapy
 Brachytherapy
 Surgery (Best Option) taking the whole thing off.

5. Choice of Treatment
The choice of treatment depends on a
number of factors including:
 Size of tumor
 Position of tumor
 Tumor stage

6. Objective of Radiation
Therapy
Radiation therapy may be used:
 to cure an illness - for example, by
destroying a tumor (abnormal tissue)
 to control symptoms - for example, to
relieve pain
 before surgery - to shrink a tumor to
make it easier to remove
 after surgery - to destroy small amounts
of tumor that may be left

7. Radiation Therapy
 There are two techniques in radiation
therapy that are used to treat cancer using
ionizing radiation:
 Radiotherapy
 Brachytherapy

8. What is Radiotherapy?
 Damaging the DNA inside cells causing them to be unable
to divide and reproduce.
 Abnormal cancer cells:
 divide more quickly than normal cells.
 are more sensitive to radiation
 die and the tumor shrinks by the time
 Normal cells
 can also be damaged by radiation
 repair themselves more effectively, as when your skin heals
itself after sunburn
 The goal of radiation therapy
 maximize the dose to abnormal cells
 minimize exposure to normal cells.
 The effects of radiation are not immediate; the treatment
benefit occurs over time.

9. External Beam Irradiation
 Photon energy (X-rays or γ-rays)
 Particle Radiation (electrons, protons, neutrons)
 Photon therapy advantages
 Skin sparing, penetration, beam uniformity
 Dual-energy linear accelerators generate:
 Low energy megavoltage X-rays (4-6 MeV)
 High energy X-rays (15-20 MeV)
 Electron beam
 Cobalt units and low-energy linacs (4-6 MeV) are used
primarily to treat bone cancer and tumors of the head,
neck, and breast.
 High-energy linacs are used to treat deep-seated tumors
of the pelvis and thorax.

10. Radiotherapy Using X-rays
 The x-rays are generated
by a linear accelerator
(linac).
 The linac fires high
energy electrons at a
metal target and when the
electrons strike the target,
x-rays are produced.
 The x-rays produced are
shaped into a narrow
beam that matches the
patient’s tumor using
movable metal shutters.
 The beam comes out of a
gantry which rotates
around the patient.

11. Radiotherapy Using Gamma
Rays
 Gamma rays are
emitted from a cobalt-
60 source – a
radioactive form of
cobalt.
 The cobalt source is
kept within a thick,
heavy metal
container.
 This container has a
slit in it to allow a
narrow beam of
gamma rays to
emerge.

12. Radiotherapy
 The apparatus is arranged so that it
can rotate around the couch (table)
on which the patient lies.
 This allows the patient to receive
radiation from different directions.
 The diseased tissue receives
radiation all of the time but the
healthy tissue receives the minimum
amount of radiation possible.
 Treatments are given as a series of
small doses because cancerous cells
are killed more easily when they are
dividing, and not all cells divide at the
same time – this reduces some of the
side effects which come with
radiotherapy.

13. Brachytherapy
 This involves placing
implants made of a
radioactive source in the
form of seeds, wires or
pellets directly into the
tumor.
 Such implants may be
temporary or permanent
depending on the implant
and the tumor itself.
 The benefit of such a
method is that the tumor
receives nearly all of the
dose whilst healthy tissue

14. Brachytherapy
 Uterus
 Cervix
 Prostate
 Intraocular
 Skin
 Thyroid
 Bone
Brachytherapy is used to treat the following cancers:

15. Brachytherapy
 Radioactive source in direct contact with tumor
 Radioactive sources are put inside the patient
 The radioactive sources are sealed in needles,
seeds, wires, or catheters, and implanted directly
into or near a tumor on a temporary, interstitial
implants, intracavitary implants or surface molds.
 Greater deliverable dose
 Continuous low dose rate
 Shorter treatment times

16. Brachytherapy
 Limitations
 Tumor must be accessible
 Well-demarcated with defined boundaries
 Cannot be the only modality for tumors with
high risk of regional lymph node metastasis

17. Brachytherapy

18. Radiobiology
 Use of high-energy radiation to treat cancer
 destroys the cancer cells' ability to reproduce, and
the body naturally gets rid of these cells.
 Radiation destroys cancer cells by damaging their
DNA.
 Cancer cells are particularly vulnerable to
radiation
 Cancer cells divide more rapidly than normal
cells.
 Normal cells are able to repair this damage
more efficiently.

19. Radiobiology
 Random cell death
 Deposition of energy & injury is random
 Same proportion of cells is damaged per
dose
 100 to 10 cell reduction = 106 to 105 cell reduction
 Larger tumors require more radiation
 105 cells = nonpalpable
 Normal tissue is affected also

20. Radiobiology
 Ionizing radiation ejects an electron from a
target molecule:
 ionizes the water in the cell and induces formation
of free radicals which cause damage of the genetic
material (DNA).
 Distributed randomly within cell
 Double-strand DNA breaks – lethal
 Cell death: no longer able to undergo unlimited
cell division
 Direct vs. Indirect injury (free radicals – O2)
 Inadequate cellular repair mechanisms implied

21. Direct vs. Indirect Action
Direct Action:
 Radiation may impact the DNA directly, causing ionization
of the atoms in the DNA molecule (“direct hit”).
Indirect Action:
 Radiation interacts with non-critical target molecules as
water.
 This results in the production of free radicals, which are
atoms or molecules that have an unpaired electron.
 Electrons like to be in pairs, thus free radicals are highly
unstable seeking out other electrons so they can become
a pair.
 These free radicals can then attack critical targets such as
the DNA, causing its ionization and a chain reaction
occurs.
 Damage from indirect action is much more common than 22

22. Direct vs. Indirect Action

23. Cancer Treatment using
Radiotherapy
 Kilovoltage X-ray units:
 Uses X-rays generated at voltages up to
500kVp
 Superficial: 50-150 KVp X-ray, useful for
irradiating tumor confined to about 5 mm depth
(~90% depth dose)
 Orthovoltage (or deep therapy): 150- 500 KVp
X-ray, Treatment to a depth of only a few
centimeters
 Megavoltage linear accelerators (LINAC)
 Magnetron
 Cyclotron
 Cobalt-60 machines

24. Diagnostic and Therapeutic X-
Ray
 Diagnostic X-ray
 uses low energy X-rays for imaging
 Therapeutic X-ray
 uses high energy X-rays to treat tumor.
 ionizes the water in the cell and induces formation of
free radicals which cause damage of the genetic
material (DNA).
 Often the damage is not enough to cause tumor cell
death. i.e. it is sublethal.
 Lethal damage occurs by repeated exposure to
radiation.
 Normal cells are also affected adversely by radiation
but have the capacity of repair, i.e., reversible.

25. Measurement of radiation
 Absorbed dose:
refers to the energy deposited at a specific point in a
medium.
Measured in the SI unit of the gray (Gy), where
1Gy =1 Joule/kg
1 Gy = 100 cGy (100 rad)
 Depth: distance beneath the skin surface where the
prescribed dose is to be delivered.
 Depth affects measurements of dose attenuation.
 Source to Skin Distance (SSD): the distance from the X-
ray source to the surface of the patient.

26. Measurement of radiation
 Percent Depth Dose (PDD): measures the
attenuation of the dose as it travels through matter.
 Is the percentage ratio of the absorbed dose at a
given depth to the absorbed dose at a fixed
reference depth usually DMAX , where dose reaches
its maximum value.
 Dependant on:
↑ Energy (Beam quality)- more penetrating- ↑ PDD
↑ Field size- more scatter- ↑ PDD
↑ SSD- ↑ PDD
↑ Depth- ↓ PDD due to dose attenuation through matter
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27. Kilovoltage X-ray units
 “Conventional” X-Ray tube with electrons accelerated by an
electric field.
 Stationary anode made of tungsten 2-3 mm thick embedded in
a large mass of copper for heat dissipation, with direct water
cooling of the copper block.
 Is sufficient (in contrast to diagnostic tubes which have a
rotating anode) as short, low-dose exposures are intended.
Also, to allow for a more compact, space-saving and less
expensive model.
Metal–ceramic tube

28. Beam Filtration
 Filtration is important to harden the beam to a desired
degree:
 Lower-energy photons are quickly attenuated in tissue and
contribute little dose at greater depths.
 Depending on the intended depth of treatment, this low-
energy component needs to be substantially reduced as it
increases dose in superficial tissue without clinical benefit.
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 A filter holder and applicator
mount are attached to the
tube housing at the beam exit
aperture.
 A filter storage box contains
interchangeable filters that
can be inserted beneath the
beam exit aperture to produce
beams of different qualities

29. Beam Collimation
 The primary collimator
usually consists of a
conical hole within a block
of lead, or other heavy
metal, set into the tube
housing.
 Secondary collimation is
accomplished using a set
of interchangeable
applicators covering the
range of available
treatment sizes.
 The main collimation from
an applicator is provided
by a diaphragm of the

30. Beam Collimation
 Applicators accurately define the
treatment area and the dose rate:
 The applicator end defines the
shape and size of the radiation
beam (field size at the skin
surface).
 The applicator end sets a fixed
distance from the source
(SSD), typically in the range of
20 to 50 cm.
 The axis of the applicator is
mechanically aligned to the
axis of the radiation beam.
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31. Dose Control
 A transmission ionization chamber is placed
downstream of the filter holder. Such
equipment is a radiation monitor to indicate
the tube output rate.
 Can be calibrated to deliver the intended dose
in terms of monitor units (MU).
 Ionization chamber shuts down beam after a
predetermined dose is given.
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32. Superficial Equipment
 Superficial X-Ray tube (Philips RT100)
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33. 34

34. Limitations of Kilovoltage X-
ray Machines
 Can not reach deep-seated tumors with
an adequate dosage of radiation.
 Do not spare skin and normal tissues:
 Beyond certain depth, the dose drop-off is
too severe to deliver adequate depth dose
without considerable overdosing of the skin
surface
Machine Learning Spring 2014 Inas A.
Yassine 35